Nonvolatile Memory with Correlated Multiple Pass Programming

ABSTRACT

A group of memory cells is programmed respectively to their target states in parallel using a multiple-pass programming method in which the programming voltages in the multiple passes are correlated. Each programming pass employs a programming voltage in the form of a staircase pulse train with a common step size, and each successive pass has the staircase pulse train offset from that of the previous pass by a predetermined offset level. The predetermined offset level is less than the common step size and may be less than or equal to the predetermined offset level of the previous pass. Thus, the same programming resolution can be achieved over multiple passes using fewer programming pulses than conventional method where each successive pass uses a programming staircase pulse train with a finer step size. The multiple pass programming serves to tighten the distribution of the programmed thresholds while reducing the overall number of programming pulses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 12/899,291, filed on Oct. 6, 2010, which is a continuationapplication of U.S. application Ser. No. 12/138,387, filed on Jun. 12,2008, now U.S. Pat. No. 7,813,172. This application is also related tothe following U.S. patent applications: U.S. application Ser. No.12/138,371, entitled “METHOD FOR INDEX PROGRAMMING AND REDUCED VERIFY INNONVOLATILE MEMORY” by Raul Adrian Cernea, filed on Jun. 12, 2008, nowU.S. Pat. No. 7,800,945; U.S. application Ser. No. 12/138,378, entitled“NONVOLATILE MEMORY WITH INDEX PROGRAMMING AND REDUCED VERIFY” by RaulAdrian Cernea, filed on Jun. 12, 2008, now U.S. Pat. No. 7,826,271. U.S.application Ser. No. 12/138,382, entitled “METHOD FOR CORRELATEDMULTIPLE PASS PROGRAMMING IN NONVOLATILE MEMORY” by Raul Adrian Cernea,filed on Jun. 12, 2008, now U.S. Pat. No. 7,796,435; U.S. patentapplication Ser. No. 11/733,694, “PREDICTIVE PROGRAMMING IN NON-VOLATILEMEMORY” filed on Apr. 10, 2007, by the same inventor as the presentapplication, now U.S. Pat. No. 7,643,348; U.S. patent application Ser.No. 11/733,706, “NON-VOLATILE MEMORY WITH PREDICTIVE PROGRAMMING” filedon Apr. 10, 2007, by the same inventor as the present application, nowU.S. Pat. No. 7,551,483; and U.S. patent application Ser. No. 12/649,184filed on Dec. 29, 2009, by the same inventor as the present application,now U.S. Pat. No. 7,965,562.

FIELD OF THE INVENTION

This invention relates generally to non-volatile semiconductor memorysuch as electrically erasable programmable read-only memory (EEPROM) andflash EEPROM, and specifically to memory and programming operations inwhich the number of program-verify operations is minimized.

BACKGROUND OF THE INVENTION

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash EEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile and retains itsstored data even after power is turned off. In spite of the higher cost,flash memory is increasingly being used in mass storage applications.Conventional mass storage, based on rotating magnetic medium such ashard drives and floppy disks, is unsuitable for the mobile and handheldenvironment. This is because disk drives tend to be bulky, are prone tomechanical failure and have high latency and high power requirements.These undesirable attributes make disk-based storage impractical in mostmobile and portable applications. On the other hand, flash memory, bothembedded and in the form of a removable card, are ideally suited in themobile and handheld environment because of its small size, low powerconsumption, high speed and high reliability features.

EEPROM and electrically programmable read-only memory (EPROM) arenon-volatile memory that can be erased and have new data written or“programmed” into their memory cells. Both utilize a floating(unconnected) conductive gate, in a field effect transistor structure,positioned over a channel region in a semiconductor substrate, betweensource and drain regions. A control gate is then provided over thefloating gate. The threshold voltage characteristic of the transistor iscontrolled by the amount of charge that is retained on the floatinggate. That is, for a given level of charge on the floating gate, thereis a corresponding voltage (threshold) that must be applied to thecontrol gate before the transistor is turned “on” to permit conductionbetween its source and drain regions.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell. When the threshold voltage is partitioned into two distinctregions, each memory cell will be able to store one bit of data.Similarly, when the threshold voltage window is partitioned into morethan two distinct regions, each memory cell will be able to store morethan one bit of data.

In the usual two-state EEPROM cell, at least one current breakpointlevel is established so as to partition the conduction window into tworegions. When a cell is read by applying predetermined, fixed voltages,its source/drain current is resolved into a memory state by comparingwith the breakpoint level (or reference current IREF). If the currentread is higher than that of the breakpoint level, the cell is determinedto be in one logical state (e.g., a “zero” state). On the other hand, ifthe current is less than that of the breakpoint level, the cell isdetermined to be in the other logical state (e.g., a “one” state). Thus,such a two-state cell stores one bit of digital information. A referencecurrent source, which may be externally programmable, is often providedas part of a memory system to generate the breakpoint level current.

In order to increase memory capacity, flash EEPROM devices are beingfabricated with higher and higher density as the state of thesemiconductor technology advances. Another method for increasing storagecapacity is to have each memory cell store more than two states.

For a multi-state or multi-level EEPROM memory cell, the conductionwindow is partitioned into more than two regions by more than onebreakpoint such that each cell is capable of storing more than one bitof data. The information that a given EEPROM array can store is thusincreased with the number of states that each cell can store. EEPROM orflash EEPROM with multi-state or multi-level memory cells have beendescribed in U.S. Pat. No. 5,172,338.

The transistor serving as a memory cell is typically programmed to a“programmed” state by one of two mechanisms. In “hot electroninjection,” a high voltage applied to the drain accelerates electronsacross the substrate channel region. At the same time a high voltageapplied to the control gate pulls the hot electrons through a thin gatedielectric onto the floating gate. In “tunneling injection,” a highvoltage is applied to the control gate relative to the substrate. Inthis way, electrons are pulled from the substrate to the interveningfloating gate.

The memory device may be erased by a number of mechanisms. For EPROM,the memory is bulk erasable by removing the charge from the floatinggate by ultraviolet radiation. For EEPROM, a memory cell is electricallyerasable, by applying a high voltage to the substrate relative to thecontrol gate so as to induce electrons in the floating gate to tunnelthrough a thin oxide to the substrate channel region (i.e.,Fowler-Nordheim tunneling.) Typically, the EEPROM is erasable byte bybyte. For flash EEPROM, the memory is electrically erasable either allat once or one or more blocks at a time, where a block may consist of512 bytes or more of memory.

The memory devices typically comprise one or more memory chips that maybe mounted on a card. Each memory chip comprises an array of memorycells supported by peripheral circuits such as decoders and erase, writeand read circuits. The more sophisticated memory devices operate with anexternal memory controller that performs intelligent and higher levelmemory operations and interfacing.

There are many commercially successful non-volatile solid-state memorydevices being used today. These memory devices may be flash EEPROM ormay employ other types of nonvolatile memory cells. Examples of flashmemory and systems and methods of manufacturing them are given in U.S.Pat. Nos. 5,070,032, 5,095,344, 5,315,541, 5,343,063, and 5,661,053,5,313,421 and 6,222,762. In particular, flash memory devices with NANDstring structures are described in U.S. Pat. Nos. 5,570,315, 5,903,495,6,046,935. Also nonvolatile memory devices are also manufactured frommemory cells with a dielectric layer for storing charge. Instead of theconductive floating gate elements described earlier, a dielectric layeris used. Such memory devices utilizing dielectric storage element havebeen described by Eitan et al., “NROM: A Novel Localized Trapping, 2-BitNonvolatile Memory Cell,” IEEE Electron Device Letters, vol. 21, no. 11,November 2000, pp. 543-545. An ONO dielectric layer extends across thechannel between source and drain diffusions. The charge for one data bitis localized in the dielectric layer adjacent to the drain, and thecharge for the other data bit is localized in the dielectric layeradjacent to the source. For example, U.S. Pat. Nos. 5,768,192 and6,011,725 disclose a nonvolatile memory cell having a trappingdielectric sandwiched between two silicon dioxide layers. Multi-statedata storage is implemented by separately reading the binary states ofthe spatially separated charge storage regions within the dielectric.

In order to improve read and program performances, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a “page” of memory elements are read orprogrammed together. In existing memory architectures, a row typicallycontains several interleaved pages or it may constitute one page. Allmemory elements of a page will be read or programmed together.

The conventional programming technique of using a series of alternatingprogram/verify cycles is to deal with the uncertainty in the programmingprocess in which the cell's threshold voltage grows fast initially inresponse to a relatively large change in V_(PGM). However, the growthslows down and eventually stops as charges programmed into the floatinggate act as a shield to diminish the effective electric field forfurther tunneling of the electrons into the floating gate. The processappears highly non-linear and hence a trial-and-error approach isemployed.

The disadvantage of the program/verify programming technique is that theverify cycle takes up time and impacts performance. The problem isexacerbated by the implementation of memory cells capable of storingmultiple bits. Essentially verify needs to be performed for each of thepossible multiple states of a memory cell. For a memory with 16 possiblememory states, this means each verify cycle may incur up to 16 sensingoperations. Thus, with increasing number of distinguishable state levelsin multi-level memory cells (“MLC”), the verify cycle of theprogram/verify scheme becomes increasingly time-consuming.

U.S. patent application Ser. No. 11/531,227, entitled, “Method forNon-volatile Memory with Linear Estimation of Initial ProgrammingVoltage” filed by Loc Tu et al on Sep. 12, 2006, now U.S. Pat. No.7,453,731, discloses a method of estimating initial programming voltagesby linear estimation. In order to achieve good programming performancefor a non-volatile memory, the initial programming voltage V_(PGM0) andthe step size must be optimally chosen at the factory. This isaccomplished by testing each page of memory cells. The word line coupledto a selected page is successively programmed by a series of voltagepulses of a staircase waveform with verifications in between the pulsesuntil the page is verified to a designated pattern. The programmingvoltage at the time the page is programmed verified will be used toestimate by linearly scaling back to the initial value of a startingprogramming voltage for the page. The estimation is further refined byusing the estimate from a first pass in a second pass. Thus,conventional alternating programming and verifications are used toestablish a final programming voltage for successfully programming apage. Then the final programming voltage is linearly scaled back toarrive at an estimated initial programming voltage for the page. Thistype of scaling is on a gross scale at a page level and does not addressthe disadvantage of conventional programming and verifying the memory inthe field on a cell by cell basis.

In particular, the conventional programming requires a verify operationin between every pulse. When the memory is partitioned into many memorystates, the verify operation must check many states in between everypulse. The number of verify operations increases with the square of thenumber of state partitions. Thus, for memory that hold 3 or more bits ofdata per cell, the number of verify operations become prohibitivelylarge.

To improve programming resolution, a conventional method is to make theprogramming pulse step size finer. However, this has the effect ofproportionally increasing the number of pulses require to programthereby increasing programming time. Furthermore, the increase inprogramming pulses will compound to the number of interleavingverifications in conventional methods.

Therefore there is a general need for high capacity and high performancenon-volatile memory. In particular, there is a need to have a highcapacity nonvolatile memory with improved programming performance wherethe aforementioned disadvantage is minimized.

SUMMARY OF INVENTION Correlated Multi-Pass Programming

In a multi-state memory, each cell can be programmed to one of themulti-states with a threshold voltage within one of predefined ranges ofthreshold voltages. In a population of such memory cells, it isdesirable to program accurately so that the various ranges of thresholdvoltages or distributions do not spread out to form indistinct ranges.One technique of tightening the distribution is to perform multipleprogramming passes, each time using a finer programming pulse step size.However, with ever finer pulse step size, the programming performancedecreases with the increase in the number of pulses.

According to another aspect of the invention, a group of memory cellsare programmed in parallel in multiple programming passes in which theprogramming voltages in the multiple passes are correlated. Eachprogramming pass employing a programming voltage in the form of astaircase pulse train with a common step size, and each successive passhas the staircase pulse train offset from that of the previous pass by apredetermined offset level. The predetermined offset level is less thanthe common step size and may be less than or equal to the predeterminedoffset level of the previous pass.

In one preferred embodiment, the predetermined offset is half of that ofthe previous pass. For example, the staircase pulse train of the secondpass is offset from the first by half a step size and the staircasepulse train of the third pass is offset from the second by a quarterstep size. In each pass the number of pulses is the same. In this way,the same programming resolution can be achieved over multiple passesusing fewer programming pulses than that the conventional method ofusing multiple passes with each pass using a programming staircase pulsetrain with finer step size.

The correlation multi-pass programming is advantageous in improving theprogramming performance by reducing the number of programming pulsesover multiple programming passes.

The multiple-pass index programming technique allows substantial savingin the number of verify operations. Similarly, the multiple-passcorrelated programming technique allows substantial saving in the numberof programming pulses required. The two techniques can be integratedtogether into a high performance, multiple-pass index and correlatedprogramming. The benefits are even more so for a memory configured tostore three or more bits of data per cell.

Additional features and advantages of the present invention will beunderstood from the following description of its preferred embodiments,which description should be taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the functional blocks of a non-volatilememory chip in which the present invention may be implemented.

FIG. 2 illustrates schematically a non-volatile memory cell.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time.

FIG. 4 illustrates an example of an NOR array of memory cells.

FIG. 5A illustrates schematically a string of memory cells organizedinto an NAND string.

FIG. 5B illustrates an example of an NAND array 200 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 5A.

FIG. 6 illustrates the Read/Write Circuits 270A and 270B, shown in FIG.1, containing a bank of p sense modules across an array of memory cells.

FIG. 7 illustrates schematically a preferred organization of the sensemodules shown in FIG. 6.

FIG. 8 illustrates in more detail the read/write stacks shown in FIG. 7.

FIGS. 9(0)-9(2) illustrate an example of programming a population of4-state memory cells.

FIGS. 10(0)-10(2) illustrate an example of programming a population of8-state memory cells.

FIG. 11 illustrates a conventional technique for programming a 4-statememory cell to a target memory state.

FIG. 12 is a table illustrating estimated numbers of programming pulsesand verifying cycles to program a page using conventional alternatingprogram/verify algorithm.

FIG. 13 is a flow diagram illustrating a general scheme of the indexprogramming method.

FIG. 14A is a flow diagram illustrating providing the program index of amemory cell according to the first implementation.

FIG. 14B is a flow diagram illustrating a second implementation ofobtaining the program index for a memory cell.

FIG. 14C is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell using a predictive functioncalibrated by one or more checkpoint.

FIG. 14D is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell according to oneembodiment.

FIG. 14E is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell according to anotherembodiment.

FIG. 15 illustrates a preferred embodiment of the predetermined functionused to provide the programming voltage needed to program the memorycell to a targeted threshold voltage level.

FIG. 16 illustrates a preferred designation for the checkpoint tocorrespond to a first programmed state above the erased state.

FIG. 17 illustrates the predictive programming employed in a firstprogramming pass and to build the program index for each cell.

FIG. 18A is a flow diagram illustrating setting a programming voltagewith step size such that each additional pulse will program a memorycell to the next memory state.

FIG. 18B illustrates schematically the threshold voltage of a memorycell undergoing the first programming pass.

FIG. 19 is a flow diagram illustrating a preferred implementation ofestablishing a program index for a memory cell.

FIGS. 20(A), 20(B) and 20(C) respectively illustrate the latch operationof FIG. 19 for a “regular” cell, a “slow” cell and a “very slow” cellshown in FIG. 18B.

FIG. 21 is a flow diagram illustrating a preferred embodiment of theindex programming method.

FIG. 22 illustrates the additional verifying and programming passesshown in STEP 820 of FIG. 21 for trimming the programmed results afterthe first pass.

FIG. 23 illustrates schematically a latch for storing a verify statusflag.

FIG. 24A is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by the use of the verify statusflag.

FIG. 24B is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by offsetting the program index forthe memory cell.

FIG. 24C is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by offsetting the pulse count.

FIG. 25 is a table illustrating estimated numbers of programming pulsesand verifying cycles to program a page using the index programmingtechnique.

FIG. 26 illustrates the application of the correlated multi-passprogramming to the index programming passes shown in FIG. 21.

FIG. 27 illustrates the tightening of the threshold voltage distributionof the memory states by using multiple-pass programming.

FIG. 28A is a table showing the number of programming pulses used in aconventional multiple-pass programming for various partitioning ofmemory states.

FIG. 28B is a table showing the number of programming pulses used in thecorrelated multiple-pass programming for various partitioning of memorystates.

FIG. 29 is a flow diagramming illustrating a multiple-pass programmingmethod employing correlated programming levels between the passes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Memory System

FIG. 1 to FIG. 10 illustrate example memory systems in which the variousaspects of the present invention may be implemented.

FIG. 11 and FIG. 12 illustrate a conventional programming technique.

FIG. 13 to FIG. 29 illustrate the various aspects and embodiments of thepresent invention.

FIG. 1 illustrates schematically the functional blocks of a non-volatilememory chip in which the present invention may be implemented. Thememory chip 100 includes a two-dimensional array of memory cells 200,control circuitry 210, and peripheral circuits such as decoders,read/write circuits and multiplexers.

The memory array 200 is addressable by word lines via row decoders 230(split into 230A, 230B) and by bit lines via column decoders 260 (splitinto 260A, 260B) (see also FIGS. 4 and 5.) The read/write circuits 270(split into 270A, 270B) allow a page of memory cells to be read orprogrammed in parallel. A data I/O bus 231 is coupled to the read/writecircuits 270.

In a preferred embodiment, a page is constituted from a contiguous rowof memory cells sharing the same word line. In another embodiment, wherea row of memory cells are partitioned into multiple pages, blockmultiplexers 250 (split into 250A and 250B) are provided to multiplexthe read/write circuits 270 to the individual pages. For example, twopages, respectively formed by odd and even columns of memory cells aremultiplexed to the read/write circuits.

FIG. 1 illustrates a preferred arrangement in which access to the memoryarray 200 by the various peripheral circuits is implemented in asymmetric fashion, on opposite sides of the array so that the densitiesof access lines and circuitry on each side are reduced in half. Thus,the row decoder is split into row decoders 230A and 230B and the columndecoder into column decoders 260A and 260B. In the embodiment where arow of memory cells are partitioned into multiple pages, the pagemultiplexer 250 is split into page multiplexers 250A and 250B.Similarly, the read/write circuits 270 are split into read/writecircuits 270A connecting to bit lines from the bottom and read/writecircuits 270B connecting to bit lines from the top of the array 200. Inthis way, the density of the read/write modules, and therefore that ofthe sense modules 380, is essentially reduced by one half.

The control circuitry 110 is an on-chip controller that cooperates withthe read/write circuits 270 to perform memory operations on the memoryarray 200. The control circuitry 110 typically includes a state machine112 and other circuits such as an on-chip address decoder and a powercontrol module (not shown explicitly). The state machine 112 provideschip level control of memory operations. The control circuitry is incommunication with a host via an external memory controller.

The memory array 200 is typically organized as a two-dimensional arrayof memory cells arranged in rows and columns and addressable by wordlines and bit lines. The array can be formed according to an NOR type oran NAND type architecture.

FIG. 2 illustrates schematically a non-volatile memory cell. The memorycell 10 can be implemented by a field-effect transistor having a chargestorage unit 20, such as a floating gate or a dielectric layer. Thememory cell 10 also includes a source 14, a drain 16, and a control gate30.

There are many commercially successful non-volatile solid-state memorydevices being used today. These memory devices may employ differenttypes of memory cells, each type having one or more charge storageelement.

Typical non-volatile memory cells include EEPROM and flash EEPROM.Examples of EEPROM cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924. Examples of flash EEPROM cells, their uses inmemory systems and methods of manufacturing them are given in U.S. Pat.Nos. 5,070,032, 5,095,344, 5,315,541, 5,343,063, 5,661,053, 5,313,421and 6,222,762. In particular, examples of memory devices with NAND cellstructures are described in U.S. Pat. Nos. 5,570,315, 5,903,495,6,046,935. Also, examples of memory devices utilizing dielectric storageelement have been described by Eitan et al., “NROM: A Novel LocalizedTrapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Letters,vol. 21, no. 11, November 2000, pp. 543-545, and in U.S. Pat. Nos.5,768,192 and 6,011,725.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current. Inone implementation the detection of the conduction current relative to athreshold current is accomplished by examining the rate the conductioncurrent is discharging through the capacitance of the bit line.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time. Thefour solid I_(D) versus V_(CG) curves represent four possible chargelevels that can be programmed on a floating gate of a memory cell,respectively corresponding to four possible memory states. As anexample, the threshold voltage window of a population of cells may rangefrom 0.5V to 3.5V. Seven possible memory states “0”, “1”, “2”, “3”, “4”,“5”, “6”, respectively representing one erased and six programmed statesmay be demarcated by partitioning the threshold window into five regionsin interval of 0.5V each. For example, if a reference current, IREF of 2μA is used as shown, then the cell programmed with Q1 may be consideredto be in a memory state “1” since its curve intersects with I_(REF) inthe region of the threshold window demarcated by VCG=0.5V and 1.0V.Similarly, Q4 is in a memory state “5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Forexample, a memory device may have memory cells having a threshold windowthat ranges from −1.5V to 5V. This provides a maximum width of 6.5V. Ifthe memory cell is to store 16 states, each state may occupy from 200 mVto 300 mV in the threshold window. This will require higher precision inprogramming and reading operations in order to be able to achieve therequired resolution.

FIG. 4 illustrates an example of an NOR array of memory cells. In thememory array 200, each row of memory cells are connected by theirsources 14 and drains 16 in a daisy-chain manner. This design issometimes referred to as a virtual ground design. The cells 10 in a rowhave their control gates 30 connected to a word line, such as word line42. The cells in a column have their sources and drains respectivelyconnected to selected bit lines, such as bit lines 34 and 36.

FIG. 5A illustrates schematically a string of memory cells organizedinto an NAND string. An NAND string 50 comprises of a series of memorytransistors M1, M2, . . . Mn (e.g., n=4, 8, 16 or higher) daisy-chainedby their sources and drains. A pair of select transistors S1, S2controls the memory transistors chain's connection to the external viathe NAND string's source terminal 54 and drain terminal 56 respectively.In a memory array, when the source select transistor S1 is turned on,the source terminal is coupled to a source line (see FIG. 5B).Similarly, when the drain select transistor S2 is turned on, the drainterminal of the NAND string is coupled to a bit line of the memoryarray. Each memory transistor 10 in the chain acts as a memory cell. Ithas a charge storage element 20 to store a given amount of charge so asto represent an intended memory state. A control gate 30 of each memorytransistor allows control over read and write operations. As will beseen in FIG. 5B, the control gates 30 of corresponding memorytransistors of a row of NAND string are all connected to the same wordline. Similarly, a control gate 32 of each of the select transistors S1,S2 provides control access to the NAND string via its source terminal 54and drain terminal 56 respectively. Likewise, the control gates 32 ofcorresponding select transistors of a row of NAND string are allconnected to the same select line.

When an addressed memory transistor 10 within an NAND string is read oris verified during programming, its control gate 30 is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND string 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effective created from the source of the individualmemory transistor to the source terminal 54 of the NAND string andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND string structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.

FIG. 5B illustrates an example of an NAND array 200 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 5A. Alongeach column of NAND strings, a bit line such as bit line 36 is coupledto the drain terminal 56 of each NAND string. Along each bank of NANDstrings, a source line such as source line 34 is couple to the sourceterminals 54 of each NAND string. Also the control gates along a row ofmemory cells in a bank of NAND strings are connected to a word line suchas word line 42. The control gates along a row of select transistors ina bank of NAND strings are connected to a select line such as selectline 44. An entire row of memory cells in a bank of NAND strings can beaddressed by appropriate voltages on the word lines and select lines ofthe bank of NAND strings. When a memory transistor within a NAND stringis being read, the remaining memory transistors in the string are turnedon hard via their associated word lines so that the current flowingthrough the string is essentially dependent upon the level of chargestored in the cell being read.

Sensing Circuits and Techniques

FIG. 6 illustrates the Read/Write Circuits 270A and 270B, shown in FIG.1, containing a bank of p sense modules across an array of memory cells.The entire bank of p sense modules 480 operating in parallel allows ablock (or page) of p cells 10 along a row to be read or programmed inparallel. Essentially, sense module 1 will sense a current I₁ in cell 1,sense module 2 will sense a current I₂ in cell 2, . . . , sense module pwill sense a current I_(p) in cell p, etc. The total cell currenti_(TOT) for the page flowing out of the source line 34 into an aggregatenode CLSRC and from there to ground will be a summation of all thecurrents in the p cells. In conventional memory architecture, a row ofmemory cells with a common word line forms two or more pages, where thememory cells in a page are read and programmed in parallel. In the caseof a row with two pages, one page is accessed by even bit lines and theother page is accessed by odd bit lines. A page of sensing circuits iscoupled to either the even bit lines or to the odd bit lines at any onetime. In that case, page multiplexers 250A and 250B are provided tomultiplex the read/write circuits 270A and 270B respectively to theindividual pages.

In currently produced chips based on 56 nm technology p>64000 and in the43 nm 32 Gbit x4 chip p>150000. In the preferred embodiment, the blockis a run of the entire row of cells. This is the so-called “allbit-line” architecture in which the page is constituted from a row ofcontiguous memory cells coupled respectively to contiguous bit lines. Inanother embodiment, the block is a subset of cells in the row. Forexample, the subset of cells could be one half of the entire row or onequarter of the entire row. The subset of cells could be a run ofcontiguous cells or one every other cell, or one every predeterminednumber of cells. Each sense module is coupled to a memory cell via a bitline and includes a sense amplifier for sensing the conduction currentof a memory cell. In general, if the Read/Write Circuits are distributedon opposite sides of the memory array the bank of p sense modules willbe distributed between the two sets of Read/Write Circuits 270A and270B.

FIG. 7 illustrates schematically a preferred organization of the sensemodules shown in FIG. 6. The read/write circuits 270A and 270Bcontaining p sense modules are grouped into a bank of read/write stacks400.

FIG. 8 illustrates in more detail the read/write stacks shown in FIG. 7.Each read/write stack 400 operates on a group of k bit lines inparallel. If a page has p=r*k bit lines, there will be r read/writestacks, 400-1, . . . , 400-r. Essentially, the architecture is such thateach stack of k sense modules is serviced by a common processor 500 inorder to save space. The common processor 500 computes updated data tobe stored in the latches located at the sense modules 480 and at thedata latches 430 based on the current values in those latches and oncontrols from the state machine 112. Detailed description of the commonprocessor has been disclosed in U.S. Patent Application PublicationNumber: US-2006-0140007-A1 on Jun. 29, 2006, the entire disclosure ofwhich is incorporated herein by reference.

The entire bank of partitioned read/write stacks 400 operating inparallel allows a block (or page) of p cells along a row to be read orprogrammed in parallel. Thus, there will be p read/write modules for theentire row of cells. As each stack is serving k memory cells, the totalnumber of read/write stacks in the bank is therefore given by r=p/k. Forexample, if r is the number of stacks in the bank, then p=r*k. Oneexample memory array may have p=150000, k=8, and therefore r=18750.

Each read/write stack, such as 400-1, essentially contains a stack ofsense modules 480-1 to 480-k servicing a segment of k memory cells inparallel. The page controller 410 provides control and timing signals tothe read/write circuit 370 via lines 411. The page controller is itselfdependent on the memory controller 310 via lines 311. Communicationamong each read/write stack 400 is effected by an interconnecting stackbus 431 and controlled by the page controller 410. Control lines 411provide control and clock signals from the page controller 410 to thecomponents of the read/write stacks 400-1.

In the preferred arrangement, the stack bus is partitioned into a SABus422 for communication between the common processor 500 and the stack ofsense modules 480, and a DBus 423 for communication between theprocessor and the stack of data latches 430.

The stack of data latches 430 comprises of data latches 430-1 to 430-k,one for each memory cell associated with the stack The I/O module 440enables the data latches to exchange data with the external via an I/Obus 231.

The common processor also includes an output 507 for output of a statussignal indicating a status of the memory operation, such as an errorcondition. The status signal is used to drive the gate of ann-transistor 550 that is tied to a FLAG BUS 509 in a Wired-Orconfiguration. The FLAG BUS is preferably precharged by the controller310 and will be pulled down when a status signal is asserted by any ofthe read/write stacks.

Examples of Multi-State Memory Partitioning

A nonvolatile memory in which the memory cells each stores multiple bitsof data has already been described in connection with FIG. 3. Aparticular example is a memory formed from an array of field-effecttransistors, each having a charge storage layer between its channelregion and its control gate. The charge storage layer or unit can storea range of charges, giving rise to a range of threshold voltages foreach field-effect transistor. The range of possible threshold voltagesspans a threshold window. When the threshold window is partitioned intomultiple sub-ranges or zones of threshold voltages, each resolvable zoneis used to represent a different memory states for a memory cell. Themultiple memory states can be coded by one or more binary bits. Forexample, a memory cell partitioned into four zones can support fourstates which can be coded as 2-bit data. Similarly, a memory cellpartitioned into eight zones can support eight memory states which canbe coded as 3-bit data, etc.

FIGS. 9(0)-9(2) illustrate an example of programming a population of4-state memory cells. FIG. 9(0) illustrates the population of memorycells programmable into four distinct distributions of thresholdvoltages respectively representing memory states “0”, “1”, “2” and “3”.FIG. 9(1) illustrates the initial distribution of “erased” thresholdvoltages for an erased memory. FIG. 9(2) illustrates an example of thememory after many of the memory cells have been programmed. Essentially,a cell initially has an “erased” threshold voltage and programming willmove it to a higher value into one of the three zones demarcated by DV₁,DV₂ and DV₃. In this way, each memory cell can be programmed to one ofthe three programmed state “1”, “2” and “3” or remain un-programmed inthe “erased” state. As the memory gets more programming, the initialdistribution of the “erased” state as shown in FIG. 9(1) will becomenarrower and the erased state is represented by the “0” state.

A 2-bit code having a lower bit and an upper bit can be used torepresent each of the four memory states. For example, the “0”, “1”, “2”and “3” states are respectively represented by “11”, “01”, “00” and‘10”. The 2-bit data may be read from the memory by sensing in“full-sequence” mode where the two bits are sensed together by sensingrelative to the read demarcation threshold values DV₁, DV₂ and DV₃ inthree sub-passes respectively.

FIGS. 10(0)-10(2) illustrate an example of programming a population of8-state memory cells. FIG. 10(0) illustrates the population of memorycells programmable into eight distinct distributions of thresholdvoltages respectively representing memory states “0”-“7”. FIG. 10(1)illustrates the initial distribution of “erased” threshold voltages foran erased memory. FIG. 10(2) illustrates an example of the memory aftermany of the memory cells have been programmed. Essentially, a cellinitially has an “erased” threshold voltage and programming will move itto a higher value into one of the three zones demarcated by DV₁-DV₇. Inthis way, each memory cell can be programmed to one of the sevenprogrammed state “1”-“7” or remain un-programmed in the “erased” state.As the memory gets more programming, the initial distribution of the“erased” state as shown in FIG. 10(1) will become narrower and theerased state is represented by the “0” state.

A 3-bit code having a lower bit and an upper bit can be used torepresent each of the four memory states. For example, the “0”, “1”,“2”, “3”, “4”, “5”, “6” and “7” states are respectively represented by“111”, “011”, “001”, “101”, “100”, “000”, “010” and ‘110”. The 3-bitdata may be read from the memory by sensing in “full-sequence” modewhere the three bits are sensed together by sensing relative to the readdemarcation threshold values DV₁, −DV₇ in seven sub-passes respectively.

Page or Word-Line Programming and Verify

One method of programming a page is full-sequence programming. All cellsof the page are initially in an erased state. Thus, all cells of thepage are programmed in parallel from the erased state towards theirtarget states. Those memory cells with “1” state as a target state willbe prohibited from further programming once their have been programmedto the “1” state while other memory cells with target states “2” orhigher will be subject to further programming. Eventually, the memorycells with “2” as a target state will also be locked out from furtherprogramming. Similarly, with progressive programming pulses the cellswith target states “3”-“7” are reached and locked out.

FIG. 11 illustrates a conventional technique for programming a 4-statememory cell to a target memory state. Programming circuits generallyapply a series of programming pulses to a selected word line. In thisway, a page of memory cells whose control gates are coupled to the wordline can be programmed together. The programming pulse train used mayhave increasing period or amplitude in order to counteract theaccumulating electrons programmed into the charge storage unit of thememory cell. A programming voltage V_(PGM) is applied to the word lineof a page under programming. The programming voltage V_(PGM) is a seriesof programming voltage pulses in the form of a staircase waveformstarting from an initial voltage level, V_(PGM0). Each cell of the pageunder programming is subject to this series of programming voltagepulses, with an attempt at each pulse to add incremental charges to thecharge storage element of the cell. In between programming pulses, thecell is read back to determine its threshold voltage. The read backprocess may involve one or more sensing operation. Programming stops forthe cell when its threshold voltage has been verified to fall within thethreshold voltage zone corresponding to the target state. Whenever amemory cell of the page has been programmed to its target state, it isprogram-inhibited while the other cells continue to be subject toprogramming until all cells of the page have been program-verified.

The conventional programming technique of using a series of alternatingprogram/verify cycles is to deal with the uncertainty in the programmingprocess in which the cell's threshold voltage grows fast initially inresponse to a relatively large change in V_(PGM). However, the growthslows down and eventually stops as charges programmed into the floatinggate act as a shield to diminish the effective electric field forfurther tunneling of the electrons into the floating gate.

The disadvantage of the program/verify programming technique is that theverify cycle takes up time and impacts performance. The problem isexacerbated by the implementation of memory cells capable of storingmultiple bits. Essentially verify needs to be performed for each of thepossible multiple states of a memory cell. For a memory with 16 possiblememory states, this means each verify step would incur at least 16sensing operations. In some other schemes it could even be a few timesmore. Thus, with the partitioning of a memory into increasing number ofstates, the verify cycle of the program/verify scheme becomesincreasingly time-consuming.

FIG. 12 is a table illustrating estimated numbers of programming pulsesand verifying cycles to program a page using conventional alternatingprogram/verify algorithm. For example, for an N-bit memory, thepartitioning is into Ns=2^(N) states. The number of program pulses is atleast the same of the number of states Ns. Some algorithm may require kprogramming passes, where k may be 1 to 4.) For multi-state memory, eachverify operation is further multiplied by 2^(N)−1, one for eachprogrammed state. Thus, the estimated number of verified is proportionalto 2^(2N), which is the square of the number of states. As can be seenfrom the table, for a 3-bit cell, the nominal number of verify cycles isalready extremely high, and that is not including additional sensingrequired in other schemes. For 4-bit cell, the number of verify cycle isprohibitive.

Thus, there is a need for a memory device with improved programmingperformance where the number of verify cycles is reduced.

Index Programming Techniques

According to one general aspect of the invention, a multiple-pass indexprogramming method operating on a group of memory cells in parallelcomprises maintaining for each cell a program index in order to provideinformation such as the last programming voltage level the cell hasreceived so that in a subsequent programming pass, programming orinhibiting programming of the cell relative to the program index can bemade.

Preferably, on each programming pass, a programming voltage as in aseries of incrementing pulses in the form of a staircase pulse train isapplied to the group of memory cells so that with increasing pulsecount, the memory cells are exposed to increasing programming voltages.In the preferred embodiment, each discrete programming voltage level isexpediently expressed as a pulse count or pulse number. Similarly, theprogram index is expressed in terms of a pulse number.

In a programming pass of the group of memory cells, the program index ofa cell in the group is used to control whether to allow or inhibitprogramming relative to each of the incrementing pulses.

FIG. 13 is a flow diagram illustrating a general scheme of the indexprogramming method.

-   -   STEP 700: Providing a group of memory cells to be programmed in        parallel, each memory cell programmable to an independent target        threshold voltage level.    -   STEP 710 is index programming which further comprises STEP 720,        STEP 730, and STEP 732    -   STEP 720: Providing a program index for each memory cell of the        group under programming, the program index of a memory cell        indicating the programming voltage level last used to program        the memory cell or a maximum programming voltage level the        memory cell is allowed to receive in a subsequent programming.        The program index is preferably implemented by additional latch        circuits co-operating with the read/write circuits.    -   STEP 730: Applying an incrementing programming voltage as a        series of incrementing voltage pulses in a programming pass to        the group of memory cells.    -   STEP 740: Inhibiting or allowing programming of a memory cell        under programming during the programming pass based on the        incrementing programming voltage level relative to the program        index of the memory cell

It will be seen that as the programming voltage increases, each memorycell of the group being programming in parallel is prevented fromover-programming after the programming voltage has reached the levelindicated by the program index of the cell. In this way, unlikeconventional programming method, it is not necessary to have a verifystep in between each and every programming pulses.

In a first implementation, the program index of a cell is obtained froman initial programming experience of the memory cell. The program indexstores the last programming voltage level or the pulse number applied tothe cell before it is program-inhibited during a programming pass. Theprogram index for each cell is established by interleaving programmingand verifying steps as in a conventional interleaving program/verifymethod. The programming for a cell in the group is inhibited after thecell has been program-verified and the last pulse number is recorded asits program index. While this implementation may incur more verifyingsteps, it is less likely to over program any cell. The program indexestablished for each cell can then be used advantageously in subsequentprogramming passes to save verity steps.

In a first implementation of providing the program index for a memorycell, the memory cell is programmed by a series of programming pulses,each pulse followed by a verify until the memory cell isprogram-verified to the target threshold voltage level. The programindex for the memory cell is set to be commensurate with the finalprogramming voltage when the memory cell is program-verified.

FIG. 14A is a flow diagram illustrating providing the program index of amemory cell according to the first implementation. Thus STEP 720′corresponding to STEP 720 shown in FIG. 13 further comprises STEP 721and STEP 722:

-   -   STEP 721: Alternately programming and verifying the memory cell        until the target threshold voltage level is program-verified.    -   STEP 722: Setting the program index to a value commensurate with        the programming voltage level at which the memory cell is        program-verified to the target threshold voltage level.

It will be seen that the first implementation is to obtain the programindex by a conventional programming technique where the memory cell isverified after each programming pulse. This method provides the mostaccurate program of a cell close to its target, but at the expense ofmany more verify operations.

In a second implementation, the program index of a cell is initially setto an estimated maximum programming voltage level for the cell toprogram close to but not exceed its target state, such as within apredetermined shortfall from the target state. As the staircase pulsetrain is applied to each cell in the group, a cell is inhibited fromfurther programming after reaching the expected maximum programmingvoltage level as indicated by its program index. Subsequent pulses ofthe staircase pulse train will have no effect on the inhibited cell. Atthe end of the programming pass, the each cell in the group will beprogrammed close to each respective target state and each program indexwill reflect the last programming voltage level each cell has received.

FIG. 14B is a flow diagram illustrating a second implementation ofobtaining the program index for a memory cell. Thus STEP 720″corresponding to STEP 720 shown in FIG. 13 comprises:

-   -   STEP 720″: Setting the program index of a memory cell to a        programming voltage level or equivalent pulse number estimated        to program the cell close to but not exceed its target state.

In a third implementation, the program index of a cell is estimated froman initial programming experience of the memory cell. In particular, thememory cell is programmed by a series of programming pulses, each pulsefollowed by a verify, from an erased state to a given threshold voltagelevel which serves as a checkpoint and which calibrates a predictivefunction from which the program index or the programming voltage levelfor a given target threshold voltage level is obtained.

FIG. 14C is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell using a predictive functioncalibrated by one or more checkpoint. Thus STEP 720″ corresponding toSTEP 720 shown in FIG. 13 comprises:

-   -   STEP 720″: Setting the program index of a memory cell by a        predictive function calibrated by one or more checkpoints.

The third implementation of obtaining the program index of cell by apredictive technique is described in more detail in connection with FIG.14D to FIG. 21.

FIG. 14D is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell according to oneembodiment. Thus STEP 720′″ corresponding to STEP 720 shown in FIG. 13further comprises STEP 723 to STEP 727.

-   -   STEP 723: Providing a predetermined predictive function for the        memory cell yielding a programming voltage level expected to        program the memory cell to a target threshold voltage level.    -   STEP 724: Designating a checkpoint of the predetermined function        for a memory cell with a designated checkpoint threshold voltage        level programmable by a corresponding checkpoint programming        voltage level    -   STEP 725: Determining the corresponding checkpoint programming        voltage value by alternately programming and verifying the        memory cell until the checkpoint threshold voltage level is        program-verified.    -   STEP 726: Calibrating the predetermined function to yield the        determined corresponding checkpoint programming voltage level        when evaluated at the checkpoint threshold voltage level.    -   STEP 727: Estimating the program index by evaluating the        predetermined function at the target threshold voltage level of        the memory cell.

In a second embodiment of providing a program index for a memory cell,multiple checkpoints are employed to improve the accuracy of the programindex.

FIG. 14E is a flow diagram illustrating a third implementation ofobtaining the program index of a memory cell according to anotherembodiment.

Thus STEP 720″ corresponding to STEP 720 shown in FIG. 13 furthercomprises STEP 728.

-   -   STEP 728: Similar to STEPs 723-727 of FIG. 14D except to use        more checkpoints to obtain a more accurate programming.        Predictive Programming from a Checkpoint

FIG. 15, FIG. 16, and FIG. 17 describe in more detail the predictiveprogramming shown in STEP 720′″ of FIG. 14A.

In a nonvolatile memory having an array of memory cells, wherein thememory cells are individually programmable to one of a range ofthreshold voltage levels, there is provided a predetermined functionthat predicts what programming voltage level needs to be applied inorder to program a given memory cell to a given target threshold voltagelevel. In this way, no verify operation needs be performed, therebygreatly improving in the performance of the programming operation.

In one embodiment, the predetermined function is approximated by alinear function, which proportionally yields a programming voltage levelfor a given targeted threshold voltage level. The linear function has aslope given by a predetermined average value applicable to thepopulation of cells of the memory array. The linear function is uniquelydetermined for the given memory cell by predetermining a checkpoint onthe linear function for the given memory cell. The checkpoint is basedon an actual programming voltage that programs the memory cell to adesignated threshold voltage level. The checkpoint preferablycorresponds to one of lowest program states of the memory cell. Thememory cell is initially programmed to the checkpoint by employing, forexample, the conventional program/verify programming technique. In thisway, the checkpoint values of the actual programming voltage necessaryto program the memory cell to the designated memory state is determined.The predetermined function is thus calibrated to yield the checkpointprogramming voltage value when evaluated at the checkpoint thresholdvoltage level before being used to determine a programming voltage valuefor programming the memory cell to the target threshold voltage level.

The predictive programming technique is advantageous in that programmingto a target state does not require verify operations. A verify operationonly needs to verify the checkpoint state rather than all the possiblestates of the memory.

FIG. 15 illustrates a preferred embodiment of the predetermined functionused to provide the programming voltage needed to program the memorycell to a targeted threshold voltage level. The predetermined functionis approximated by a linear function where the targeted threshold levelV_(T) is given as a function of the programming voltage V_(PGM) by therelation:

V _(T)(V _(PGM))=<Slope>V _(PGM) +V _(T)(0)  Equation (1)

(where <Slope>=ΔV_(T)/ΔV_(PGM))

Conversely,

V _(PGM)(V _(T))=1/<Slope>[V _(T) −V _(T)(0)];  Equation (2)

In the preferred embodiment, the mean <Slope> can be predetermined bytesting at the factory samples from similar production batches. Forexample, the testing may yield <Slope> to be 0.9 on average with astandard deviation of about 0.1. The V_(T)(0) is cell-dependent and ispredetermined by a checkpoint from each memory cell prior to apredictive programming of each cell. Once the <slope> and V_(T)(0) areknown, the predetermined function for the memory cell is defined andEquation (2) can be used to obtain the programming voltage level neededto program to a targeted threshold voltage level.

In general the predetermined function need not be approximated by alinear function. If the predetermined function is to accurately cover awide range of threshold voltage levels, it can be determined by testingthe production batch at the factory and modeled by some suitablefunction.

Checkpoint Calibration of the Predictive Function for Each Memory Cell

The V_(T)(0) in Equation (1) or (2) is cell-dependent and ispredetermined by designating a checkpoint threshold voltage slightlyabove that of the erased state and actually alternately programming andverifying in between pulses a given cell to the checkpoint. In this way,the actual programming voltage needed to program the given cell to thecheckpoint threshold voltage is known. This actual coordinate is thenused to solve for V_(T)(0) in Equation (2).

FIG. 14A, STEP 722, STEP 723 and STEP 724 illustrate a general principleof calibrating the predetermined function for a memory cell using acheckpoint of the function.

FIG. 16 illustrates a preferred designation for the checkpoint tocorrespond to a first programmed state above the erased state. As willbe seen in the description in the next section, when the programmingpulse train has a step size that enable each pulse to program a cell toa next memory state, the checkpoint will serve as a calibrated basestate. Obviously, if the program data for a cell requires the cell toremain in the erased state, no checkpoint will be necessary.

STEP 724′: Designating the threshold voltage level of a first programmedmemory state as a checkpoint of the predetermined function for a memorycell.

Thus, the checkpoint(0) for the memory cell is designated to be at athreshold voltage level (checkpoint threshold voltage level) slightlyhigher than that considered to be associated with the erased state. Inthe first phase of the first programming pass, a series of increasingprogramming voltage pulses is applied to program the memory cell towardthe checkpoint threshold voltage level. The programming mode can be theconventional one of alternately programming and verifying until thecheckpoint threshold voltage level is program-verified. Once the set ofcoordinates [V_(PGM), V_(T)]_(Checkpoint(0)) for Checkpoint(0) is known,the predetermined function (see FIG. 15) in the form of Equation (2) canbe solved for V_(T)(0) and be completely specified.

After the predetermined function in the form of Equation (2) isspecified, the memory cell can subsequently be programmed in the secondphase in the predictive mode using the predetermined function to providean estimated programming voltage level for a targeted threshold voltagelevel or for a targeted memory state.

Predictive programming calibrated by one or more checkpoint is alsodisclosed in U.S. patent application Ser. No. 11/733,694, “PREDICTIVEPROGRAMMING IN NON-VOLATILE MEMORY” filed on 10 Apr. 2007 by the sameinventor as the present application, now U.S. Pat. No. 7,643,348, and inU.S. patent application Ser. No. 11/733,706, “NON-VOLATILE MEMORY WITHPREDICTIVE PROGRAMMING” filed on 10 Apr. 2007 by the same inventor asthe present application, now U.S. Pat. No. 7,551,483. The entiredisclosures of the two above-mentioned applications are incorporatedherein by reference.

FIG. 17 illustrates the predictive programming employed in a firstprogramming pass and to build the program index for each cell. The firstprogramming pass is in two phases. In the example shown, the first phaseprograms the memory cells and maintains a program index using thepredictive programming method of the third implementation (see FIG.14C.) The predictive programming employs a predetermined function foreach cell which provides an estimated programming voltage needed toprogram a given cell to a given target state.

The first phase of the first programming pass is to calibrate thepredetermined function for each cell according to the programmingcharacteristic of each cell. This is accomplished by alternatelyprogramming/verifying each cell to a designated threshold voltage orcheckpoint. The checkpoint is preferably at a threshold voltage adjacentthat of the erased state so the alternately programming and verifyingtypically involve relatively few pulses. Each verify step in betweenpulses need only sense one demarcation value for the checkpoint.

In phase two, each cell will continue to be programmed starting from thecheckpoint, which is at a known position from the next memory state.Hence the predetermined function will be able to predict the programmingvoltage expected to program the cell to a given target state withouthaving to verify in between pulses as in the conventional trail anderror method. The program index for each cell will be the lastprogramming voltage level or pulse number used to program the cell inthe first programming pass.

Programming Voltage as a Pulse Train with Predetermined Step Size

In a preferred embodiment, the programming voltage step size is adjustedsuch that each additional pulse will program the memory cell to the nextmemory state. For example of a memory cell with 16 possible memorystates, the pulse size may be 300 mV. In this way, one additional pulsewill program the memory to State(1), another additional pulse willprogram the memory to State(2), etc. Thus, programming to a given memorystate can be reduced to counting the number of states from State(0) andsupplying the same number of pulses. For example, a flag may be set oncein State(0) and thereafter the memory cell can be programmed by a numberof pulses same as the number of states the target state is away fromState(0).

Other programming pulse sizes are possible. For example, for the memorycell with 16 possible memory states, the pulse size may be 150 mV. Inthat case, it will take two pulses to program from one memory state tothe next adjacent memory state. This will provide finer resolution inthe programming, which is useful in some implementations where a marginfrom the targeted threshold is employed.

FIG. 18A is a flow diagram illustrating setting a programming voltagewith step size such that each additional pulse will program a memorycell to the next memory state. The STEP 710 shown in FIG. 13 furtherincludes:

-   -   STEP 712: Providing a programming voltage having an amplitude        incrementing with time in the form of a pulse train with        incrementing amplitude.    -   STEP 714: Adjusting the amplitude increment between pulses such        that a memory cell is programmed from one programmed memory        state to a next programmed memory state by a successive pulse.

FIG. 18B illustrates schematically the threshold voltage of a memorycell undergoing the first programming pass. The memory cell starts offin an erased state which may in any one of low-lying threshold voltagelevels. During the initial programming phase, a series of program/verifycycles (e.g., a total of x program pulses plus n*x verifying steps) willprogram the memory cell from the erase state to State(0). In general,the x for each memory cell is independent of each other. Due to howdeeply erased the individual cells were and other factors, theindividual cells may differ by the number of programming pulses toarrive at a designated checkpoint. For example, a “slow” cell which hasa threshold voltage lower will take more pulses to get to State(0) thana “regular” cell with a higher threshold voltage. A “very slow” cellwhich is deeply erased will have a threshold voltage even lower and willtake make programming pulses to bring it to State(0). Once, the memorycell is in State(0), predictive programming mode commences and eachadditional pulse will program the memory cell to the next memory state.

FIG. 19 is a flow diagram illustrating a preferred implementation ofestablishing a program index for a memory cell. The program index ismaintained in one of the data latches 430 associated with the memorycell as shown in FIG. 8. The STEP 720 shown in FIG. 13 further includes:

-   -   STEP 752: Providing latches for storing a program index for the        memory cell.    -   STEP 754: Storing in the latches initially the target state in        the form of a number of pulses expected to program the memory        cell from a checkpoint state to the target state. For example,        if the target state is State(5), then the value “5” will be        stored in the latches (binary value 0101).    -   STEP 756: Computing the program index for the memory cell by        accumulating in the latches the number pulses required to        program the memory cell from an erased state to the checkpoint        state, the program index indicating the number of pulses        expected to program the memory cell to the target state. For        example, each time a pulse is applied to the memory cell in        programming it from the erased state to the checkpoint, the        program index in the latch is incremented by one.

FIGS. 20(A), 20(B) and 20(C) respectively illustrate the latch operationof FIG. 19 for a “regular” cell, a “slow” cell and a “very slow” cellshown in FIG. 18B.

FIG. 20(A) illustrates the latch operation for computing a program indexfor the example “regular” memory cell shown in FIG. 18B. The “regular”memory cell has been erased to a threshold voltage that lies near themiddle of the range of the threshold voltages of the erased population.The memory cell is to be programmed to State(3) as indicated by the datain a target state latch. Accordingly, the data latches for maintain theprogram index are initially set to “3”. With every programming pulse toget the memory cell from the erased state to the checkpoint state(0),the value in the data latches is incremented by one. Increment stopswhen the checkpoint is program-verified. In this example, this happensafter one pulse and the program index in the latches has incremented to“4”. This means that this cell expects four pulses to program toState(3). To program the cell from the checkpoint to State(3),additional three pulses to bring the total to four pulses are applied.After the cell has been subject to the number of pulses equal to theprogram index, the cell is inhibited from programming while other cellsin the page may continue to be programmed. This is indicated by aProgram/Inhibit status going from “P” to “I”.

FIG. 20(B) illustrates the latch operation for computing a program indexfor the example “slow” memory cell shown in FIG. 18B. The “slow” memorycell has been erased to a threshold voltage that lies lower than themiddle of the range of the threshold voltages of the erased population.The memory cell is also to be programmed to State(3) as indicated by thedata in a target state latches. Accordingly, the data latches formaintain the program index are initially set to “3”. With everyprogramming pulse to get the memory cell from the erased state to thecheckpoint state(0), the value in the data latch is incremented by one.Increment stops when the checkpoint is program-verified. In thisexample, this happens after two pulses and the program index in thelatch has incremented to “5”. This means that this cell expects fivepulses to program to State(3). To program the cell from the checkpointto State(3), additional three pulses to bring the total to five pulsesare applied. After the cell has been subject to the number of pulsesequal to the program index, the cell is inhibited from programming whileother cells in the page may continue to be programmed. This is indicatedby a Program/Inhibit status going from “P” to “I”.

FIG. 20(C) illustrates the latch operation for computing a program indexfor the example “very slow” memory cell shown in FIG. 18B. The “veryslow” memory cell has been erased to a threshold voltage that lies inthe lower tail end of the range of the threshold voltages of the erasedpopulation. The memory cell is also to be programmed to State(3) asindicated by the data in a target state latches. Accordingly, the datalatch for maintain the program index is initially set to “3”. With everyprogramming pulse to get the memory cell from the erased state to thecheckpoint state(0), the value in the data latches is incremented byone. Increment stops when the checkpoint is program-verified. In thisexample, this happens after four pulses and the program index in thelatches has incremented to “7”. This means that this cell expects sevenpulses to program to State(3). To program the cell from the checkpointto State(3), additional three pulses to bring the total to five pulsesare applied. After the cell has been subject to the number of pulsesequal to the program index, the cell is inhibited from programming whileother cells in the page may continue to be programmed

Subsequent Programming Passes with Index Programming to Improve ProgramAccuracy and to Tighten Threshold Distribution

According another general aspect of the invention, a multiple-pass indexprogramming method operating on a group of memory cells in parallelincludes an initial programming pass and the building of a program indexfor each cell. The initial programming pass is followed by a verify stepand additional programming passes to trim any short-falls by the initialpass. By using index programming, the multiple-pass programming isperformed with much reduced number of verify operations.

The first programming pass, while building the program index for eachcell, preferably also programs each cell of the group to within ashortfall close to its respective target state. Then in one or moresubsequent programming pass, each of the cells is further programmedfrom its shortfall to its target state. It is preferably accomplished bya verify step before each subsequent programming pass but not betweeneach pulse in a pass. If a cell is not yet verified, it is enabled foradditional programming in the next programming pass. The program indexfor a cell at the end of a programming pass indicates the lastprogramming voltage level the cell has received. If the verify stepreveals the cell as not verified to its target state, the program indexwill be incremented by a predetermined amount to provide the expectedmaximum programming voltage allowed in the next programming pass inorder to program the cell towards its target state. In the preferredembodiment, the program index is expressed in terms of a pulse numberand is incremented by one. In the next programming pass, the memory cellwill then be subject to the next pulse based on its updated programindex.

During the next programming pass, a verified cell is inhibited fromfurther programming. An unverified cell is enabled to be programmed byone pulse beyond the one in the last programming pass. The verify stepand programming pass are repeated until all the cells in the group areverified to their respective target states. In this way, it is possibleto program a page of memory cells in parallel accurately to theirrespective target states by applying the entire run of the pulse trainbefore performing a verify step.

The advantage of index programming is that the group of cells can beprogrammed without the need for a verify step in between eachprogramming pulse of the programming pass. Index programming willgreatly improving the performance of the programming operation.

FIG. 21 is a flow diagram illustrating a preferred embodiment of theindex programming method. The method comprises a first programming passSTEP 810 for establishing the program index for each cell followed byadditional passes STEP 820 of verifying and index programming to programthe cells to their respective target states.

-   -   STEP 800: Providing a group of memory cells to be programmed in        parallel, each memory cell programmable to a respective target        state by a series of incrementing programming voltage pulses.    -   STEP 810: Building a program index for each cell of the group        during an initial programming pass, the program index storing        the last programming voltage level experienced by each cell in        terms of a pulse number.    -   STEP 820 is to verify after a programming pass and update the        program index for a next programming pass. It further comprises        STEP 822, STEP 824, STEP 826 and STEP 828:    -   STEP 830: Verifying the memory cells in the group.    -   STEP 840: Is each memory cell in the group verified to its        respective target state? If verified, proceeding to STEP 870;        otherwise, proceeding to STEP 850.    -   STEP 850: Increment the program index of each unverified memory        cell by one.    -   STEP 860: Programming each unverified memory cell with a        programming pulse selected by each program index. In the        preferred embodiment, the programming pulse selected has the        same pulse number as that indicated by the program index.        Proceeding to STEP 830 for another programming pass.    -   STEP 870: All memory cells of the group verified to have been        programmed to their respective target states.

The index programming method illustrated in FIG. 13 and FIG. 21 arepreferably implemented in the state machine 112 (see FIG. 1) in thecontrol circuitry 110 that controls memory operations of the memoryarray 200.

FIG. 22 illustrates the additional verifying and programming passesshown in STEP 820 of FIG. 21 for trimming the programmed results afterthe first pass. After a first shot at the target state in the firstprogramming pass, each memory cell is checked by verification. The firstprogramming pass tends to under shoot the target state. If any cellfails to verify to its target state, it is enabled for incrementalprogramming in a second programming pass. This verifying and programmingprocess is repeated until all the cells in the page are verified totheir respective target state. In this way, by trimming the programmedresult of a previous pass, a cell is able to converge accurately to itstarget state. Typical, one or two trimming passes are needed.

FIG. 23 illustrates schematically a latch for storing a verify statusflag. In a preferred embodiment, a latch 432, which is part of the datalatches 430 shown in FIG. 8, is used to store a verify status bit. Forexample, when a cell is verified, the verify status bit in the latch 432is set to “0”. This flag will cause the control logic to inhibit furtherprogram/verify operation on this cell. On the other hand, if the cellfails to verify, the flag will cause the control logic to allowadditional programming on the cell in the next programming pass. Aconventional implementation of a verify status flag is to indicate aprogram-inhibit through target change. In that case, when a cellverifies, the target data is programmed into the cell and is no longerneeded. Thus, the data value in the data latch indicating the targetdata is reset from a “Target code” to an “Erase code” to designate thestatus that the cell is verified. In the present invention, because ofthe need of the target data in subsequent programming pass, it isretained in the data latch. Instead the verify status is stored in theverify status flag.

FIG. 24A is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by the use of the verify statusflag. The following STEP 842 and STEP 844 take place while performingSTEP 840 in FIG. 21.

STEP 842: Setting a verify status flag in the latch according to theverified outcome.

-   -   STEP 844: Responsive to the verify status flag indicating that        the memory cell is not verified, proceeding to STEP 850        otherwise proceeding to STEP 870 of FIG. 21.

In a second preferred embodiment, the unverified memory cell is enabledfor further trim programming by offsetting higher the program index forthe memory cell by a predetermined number. In most cases, thepredetermined number in the offset is one. In this way, in the nextprogramming pass, the memory cell will be programmed by an additionalpredetermined number of pulses.

FIG. 24B is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by offsetting the program index forthe memory cell. STEP 850 of FIG. 21 is replaced by STEP 850′.

-   -   STEP 850′: When the memory cell is not verified, incrementing        the program index for the memory cell by a predetermined number        so that in a subsequent programming pass, the memory cell is        enabled to be subject to an additional predetermined number of        pulses.

In a third preferred embodiment, the unverified memory cell is enabledfor further trim programming by offsetting lower the pulse count by apredetermined number in the next programming pass. In this way, thememory cell will be programmed by an additional predetermined number ofpulses.

FIG. 24C is a flow diagram illustrating a method of enabling unverifiedmemory cells for further programming by offsetting the pulse count. STEP850 of FIG. 21 is replaced by STEP 852.

-   -   STEP 852: When the memory cell is not verified, decrementing the        programming pulse count by a predetermined number so that in a        subsequent programming pass, the memory cell is enabled to be        subject to an additional predetermined number of pulses.

FIG. 25 is a table illustrating estimated numbers of programming pulsesand verifying cycles to program a page using the index programmingtechnique. For example, for an N-bit memory, the partitioning is intoNs=2^(N) states. The number of program pulses is at least the same ofthe number of states Ns. Estimates are given for the number of pulsesand verifies for 1.1) program-verified to the checkpoint, 1.2)predictive programming from the checkpoint to the target state, and 2)one or more trimming passes. The last column in FIG. 12 shows theestimate for the total number of verifies. It can be seen thatessentially, it is proportional to the number of memory states. Thisattribute can be compared to that from using the conventional methodshown in FIG. 12, where the total number of verifies is proportional tothe square of the number of states. For example, for a memory with 3-bitmemory cells, the total number of verifies is estimated to be about 18as compared to the conventional 56. The saving is even more dramatic for4-bit memory where the total number of verifies is 34 compared to 240.

Correlated Multi-Pass Programming

The index programming method illustrated in FIG. 13 requires multipleprogramming passes. A first pass for indexing and predictive programmingis very likely followed by one or two index programming passes to trimthe programmed threshold closer to the target state. The number ofpulses in each programming pass is at least equal to the number ofmemory states. This will give a rough granularity with each pulseincreasing the threshold voltage of a cell by an amount equivalent tothe separation between two states. As a result, the thresholddistribution for each memory state (see for example FIG. 10) will bespread out.

With current algorithms, for obtaining a tighter threshold voltagedistribution for each memory state, it is possible to use finer andfiner step size with each pass. For example, in the first trimming, thepulse step size can be twice as fine compared to that used in thepredictive programming. Similarly, in the second trimming, the pulsestep size can be twice as fine compared to that used in the firsttrimming, and so on. However, each time the step size is reduced byhalf, the number of pulses and therefore the programming time willdouble.

According to another aspect of the invention, a group of memory cellsare programmed in parallel in multiple programming passes in which theprogramming voltages in the multiple passes are correlated. Eachprogramming pass employs a programming voltage in the form of astaircase pulse train with a common step size, and each successive passhas the staircase pulse train offset from that of the previous pass by apredetermined offset level. The predetermined offset level is less thanthe common step size and may be less than or equal to the predeterminedoffset level of the previous pass.

In one preferred embodiment, the predetermined offset is half of that ofthe previous pass. For example, the staircase pulse train of the secondpass is offset from the first by half a step size and the staircasepulse train of the third pass is offset from the second by a quarterstep size. In this way, the same programming resolution can be achievedover multiple passes using few programming pulses than that theconventional method of using multiple passes with each pass using aprogramming staircase pulse train with finer step size.

FIG. 26 illustrates the application of the correlated multi-passprogramming to the index programming passes shown in FIG. 21. In thatregard, FIG. 26 also shows the trimming programming passes 2) and 3)that follows from the first programming pass such as that shown in FIG.17 and FIG. 22. The staircase pulse trains used in the three passes allhave the same step size. The staircase pulse train used in the firstprogramming pass 1) has an initial programming voltage of V_(PGM0). Onthe other hand, the staircase pulse train used in the second programmingpass 2) has an initial programming voltage of V_(PGM1) where V_(PGM1) iscorrelated to V_(PGM0) such that V_(PGM1)=V_(PGM0)+ΔV_(PGM1). In apreferred embodiment, ΔV_(PGM1)=half step size.

Similarly, the staircase pulse train used in the third programming pass3) has an initial programming voltage of V_(PGM2) where V_(PGM2) iscorrelated to V_(PGM1) and V_(PGM0) such thatV_(PGM2)=V_(PGM0)+ΔV_(PGM2)=V_(PGM1)+ΔV_(PGM12). In a preferredembodiment, ΔV_(PGM2)=¾ step size, or ΔV_(PGM12)=¼ step size.

Thus, the correlated multi-pass programming employs the same staircasepulse train for programming each pass, except the DC level of the entirestaircase pulse is shifted higher by a predetermined amount with eachpass. In the preferred embodiment, the second pass is shifted by half astep size and the third pass is shifted by a quarter step size relativeto the previous pass. The programming employing these three correlatedprogramming voltage waveforms yields the same resolution as threeconventional single-pass programming where each pass uses a staircasewaveform half the step size from that of the previous pass.

FIG. 27 illustrates the tightening of the threshold voltage distributionof the memory states by using multiple-pass programming. The lower edgeof each distribution is tightened with every pass.

FIG. 28A is a table showing the number of programming pulses used in aconventional multiple-pass programming for various partitioning ofmemory states. It will be seen that the number of pulses is (2⁰+2¹+ . .. +2^(P-1))×2^(N), where P is the number of programming pass. Forexample, for 3-pass programming, a 3-bit cell will require 56 pulses anda 4-bit cell will require 112 pulses.

FIG. 28B is a table showing the number of programming pulses used in thecorrelated multiple-pass programming for various partitioning of memorystates. It will be seen that the number of pulses is just P×2^(N). Forexample, for 3-pass programming, a 3-bit cell will require 24 pulses anda 4-bit cell will require 48 pulses, which are much less than thatrequired by the conventional multiple-pass programming shown in FIG.28A.

FIG. 29 is a flow diagramming illustrating a multiple-pass programmingmethod employing correlated programming levels between the passes.

-   -   STEP 960: Providing a programming voltage incrementing with time        for a finite period in the form of a staircase pulse train with        a given step size.    -   STEP 970: Programming a group of memory cells in a predetermined        number of multiple programming passes, each successive        programming pass having the staircase pulse train applied to        program the group of the memory cells and wherein each        successive programming pass has the staircase pulse train offset        from the staircase pulse train of the previous programming pass        by a predetermined offset level.    -   STEP 980: Programming done for the group.

The multiple-pass index programming technique allows substantial savingin the number verify operations. Similarly, the multiple-pass correlatedprogramming technique allows substantial saving in the numberprogramming pulses required. The two techniques can be integratedtogether into a high performance, multiple-pass index and correlatedprogramming. The benefits are even more so for a memory configured tostore three or more bits of data per cell.

All patents, patent applications, articles, books, specifications, otherpublications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of a term between any of the incorporatedpublications, documents or things and the text of the present document,the definition or use of the term in the present document shall prevail.

Although the various aspects of the present invention have beendescribed with respect to certain embodiments, it is understood that theinvention is entitled to protection within the full scope of theappended claims.

It is claimed:
 1. A nonvolatile memory, comprising: an array of memorycells, wherein each memory cell is programmable to a respective targetstate; read/write circuits for reading and programming a group of memorycells in parallel; said read/write circuits performing programming thatcomprises: providing a programming voltage incrementing with time for afinite period in the form of a staircase pulse train with a given stepsize; and programming the group of memory cells in multiple programmingpasses, and wherein the staircase pulse trains of successive programmingpasses are offset from each other by predetermined offset levels.
 2. Thenon-volatile memory as in claim 1, wherein: the predetermined offsetlevel in each programming pass is less than the given step size and lessthan or equal to the predetermined offset level of a previousprogramming pass.
 3. The non-volatile memory as in claim 1, wherein: themultiple programming passes comprises: a first programming pass using afirst staircase pulse train; a second programming pass using a secondstaircase pulse train similar to the first staircase pulse train butoffset from the first staircase pulse train by one half of the stepsize.
 4. The non-volatile memory as in claim 3, further comprising: athird programming pass using a third staircase pulse train similar tothe second staircase pulse train but offset from the second staircasepulse train by one quarter of the step size.
 5. The non-volatile memoryas in claim 1, further comprises: a program index for each memory cellof the group under programming, the program index of a memory cellindicating the last programming voltage level used to program the memorycell; said read/write circuits performing programming that comprises:applying the programming voltage as a series of incrementing voltagepulses in a programming pass to the group of memory cells; and allowingprogramming or inhibiting programming of a memory cell during theprogramming pass according to the program index of the cell.
 6. Thenon-volatile memory as in claim 5, wherein: said read/write circuitsapplying a programming voltage as a series of incrementing voltagepulses is performed without a verify step on the group of memory cellsin between the voltage pulses during the programming pass.
 7. Thenon-volatile memory as in claim 5, wherein: the predetermined offsetlevel in each programming pass is less than the given step size and lessthan or equal to the predetermined offset level of a previousprogramming pass.
 8. The non-volatile memory as in claim 5, wherein: themultiple programming passes comprises: a first programming pass using afirst staircase pulse train; a second programming pass using a secondstaircase pulse train similar to the first staircase pulse train butoffset from the first staircase pulse train by one half of the stepsize.
 9. The non-volatile memory as in claim 8, further comprising: athird programming pass using a third staircase pulse train similar tothe second staircase pulse train but offset from the second staircasepulse train by one quarter of the step size.
 10. The non-volatile memoryas in claim 1, wherein each memory cell has a charge storing elementwhich is a floating gate of a field effect transistor.
 11. Thenon-volatile memory as in claim 1, wherein each memory cell has a chargestoring element which is a dielectric layer in a field effecttransistor.
 12. The non-volatile memory as in claim 1, wherein thenonvolatile memory has memory cells with a NAND structure.
 13. Thenon-volatile memory as in claim 1, wherein the non-volatile memory is aflash EEPROM.
 14. The non-volatile memory as in claim 1, wherein thenonvolatile memory is embodied in a memory card.
 15. A non-volatilememory as claim 1, wherein the memory cells under programming eachstores more than one bit of data.